1. Field of the Invention
The present invention relates to a numerical control apparatus capable of altering control parameters, which the apparatus, by appropriately altering the parameters used for control of feed shafts and a spindle of a machine tool, can provide optimum control at all times.
2. Description of the Related Art
FIG. 1 is a block diagram of an embodiment of a known numerical control apparatus for a machine tool. An actuating command means 1 calculates actuating commands MX, MY and MM for feed shafts (X-axis and Y-axis) and a spindle, respectively, and sends the commands to motor control means 2X, 2Y and 2M respectively. Since all of the axes are controlled in essentially the same fashion, only the case of the X-axis will be explained. The actuating command means 1 calculates the actuating command MX for the X-axis and sends it to the X-axis motor control means 2X, which receives it and then calculates current command SIX, which it sends to an X-axis current control means 3X. Based on the current command SIX, the X-axis current control means 3X generates an output current IX and drives an X-axis motor 4X. An X-axis position detector 5X, which is mechanically connected to the X-axis motor 4X, detects the position of the X-axis motor 4X and sends position detection value PX to the X-axis motor control means 2X. Next, the X-axis motor control means 2X, using the position detection value PX, controls the position of the X-axis motor 4X. The X-axis motor 4 is coupled to a table, a slide, etc. of the machine tool via a ball screw and so on and drives them, but these are not shown in FIG. 1. Control parameters such as a gain of a position loop, a proportional gain and an integral gain of a speed loop, an acceleration/deceleration time constant; and a cut-off frequency of a speed detection filter are set in the X-axis motor control means 2X and/or the X axis current control means 3X and prescribe the operation thereof.
Since the control state of the feed shafts and/or the spindle of the machine tool or the like is always varying, the optimum control parameters are also always varying. For example, since the acceleration/deceleration time constants of the feed shafts are limited by the weight of the work to be machined which is loaded on the table, if the weight changes the limitation on the acceleration/deceleration time constant also changes. Further, since the weight of the work to be machined varies during the machining, it is necessary to change the acceleration/deceleration time constants. Also, other control parameters may vary as a result of change in the weight of the work. For example, the whole weight of the table changes due to the weight changing of the work, and therefore the natural vibration frequency of the table as a whole changes. Since vibration may be generated in the table during the machining if the natural vibration frequency varies, there is a problem of degree of machining precision. In this case, it is necessary to change the control parameters such as the gain of the speed loop and the cut-off frequency of the speed detection filter in order to restrain the vibration. In addition, the machining state and environment have a great effect on control performance, i.e. on the machining precision and efficiency. In known numerical control apparatus, there is a problem of lowering of the machining precision and efficiency, because an appropriate control state cannot be maintained and therefore the control parameters which are appropriate in relation to changes in the machining state and environment and balanced for each axis cannot be maintained.